1. Field of the Invention
This invention relates to internal combustion and Stirling engines and more specifically to regenerated internal combustion engines.
2. Description of the Prior Art
The engine of the present invention has roots in two directions. First as an internal combustion engine, it is closely related to Otto and Diesel engines. Secondly, since it has a regenerator it is also closely related to the Stirling engine: combining features from these engine types gives an engine which has the high efficiency of the Stirling engine and the simplicity of construction of the internal combustion engine. This kind of engine was basically conceived by Joseph Hirsch and patented in U.S. Pat. No. 155,087, issued in 1874.
The Stirling engine, which is the precursor of all subsequent regenerated heat engines, was invented and patented in 1816 in the United Kingdom by Rev. Robert Stirling. The Stirling engine originally consisted of a working piston and cylinder, and a displacer piston, with suitable means to drive them in phased reciprocating motion, hot and cold side heat exchangers and a regenerator. Other arrangements of the working parts were later devised, such as the engine patented in 1876 by Alexander Rider, which used a second working piston to replace the dispenser piston of the original Stirling engine. During the latter portion of the nineteenth century the Stirling engine was a practical alternative to the steam engine for low power applications where safety and simplicity of operation were important. The Stirling engines of this era tended to be relatively bulky and heavy relative to their power output, but this was due principally to the primitive state of the heat transfer technology which was used in the heat exchangers. When Joseph Hirsch invented a modified Stirling engine in which internal combustion replaced the hot side heat exchanger he eliminated these limiting features of the engine. Unfortunately, at essentially the same time Nicolaus Otto invented the four stroke cycle internal combustion engine. Since Otto was employed by a firm whose business was the manufacture of gas engines, his new design was quickly put into production, and because of its simplicity, high power and low cost it has dominated the transportation industry for the past three-quarters of a century.
When the Otto engine and the Hirsch engine are compared using modern thermodynamic analysis, it can be shown that the Hirsch engine is theoretically capable of giving over twice the thermal efficiency which is possible using the Otto cylce; however, by the time that thermodynamic theory had advanced to the point where it was possible to make this comparison, the Hirsch engine had fallen into obscurity, and apparently there has never been a published description of its thermodynamic cycle.
The Hirsch engine has been intermittently rediscovered and improved over the past half century, as evidenced by the patents of Kenneth Cowans (U.S. Pats. Nos. 4,004,421 and 3,379,026), Konstantin Pattas (U.S. Pat. No. 3,777,718) and Thomas Stockton (U.S. Pat. No. 4,074,533), and by the published research studies of E. G. Hurley, "Tests on a Twin Piston Stirling Cycle Engine, Using Internal Combustion", Report K. 121, Shell Thornton Research Center, Chester, U.K. (1954) and E. H. Otten, "Tests on a Displacer-type Stirling Engine, Using Internal Combustion", Report K. 140, Shell Thornton Research Center, Chester U.K. (1956). Apparently none of these improvements and disclosures has been sufficient to threaten the entrenched position of the Otto engine.
The starting point for all of these engines is the classic Stirling cycle which is completed in one revolution of the crankshaft. The ninety degree phase difference between the pistons (in a Rider configuration 2-piston engine) means that in one rotation of the crankshaft the pistons move in the same direction for two quarter revolutions, and move in opposite directions for two quarter revolutions. Each of these quarter revolutions of the crankshaft is used for a different thermodynamic event. In the first quarter revolution the pistons both move upward, compressing the working fluid. In the next quarter revolution, in the classic external combustion Stirling engine, the compressed working fluid is moved through the cold side heat exchanger which removes the low-temperature waste heat generated by compression, through the regenerator, which returns high-temperature heat stored from the previous cycle, and through the hot side heat exchanger which adds high-temperature heat to bring the working fluid to its peak cycle temperature. In the expansion quarter revolution, the pistons both move downward, extracting work from the high-temperature, high pressure gas. In the next quarter revolution the working fluid is again moved through the hot side heat exchanger, which replaces heat lost in the power stroke, through the regenerator, which stores the high-temperature heat, and through the cold side heat exchanger which brings the gas back to the minimum cycle temperature prior to the next compression.
In all the prior art internal combustion Stirling engines the hot side heat exchanger is eliminated, and its function is replaced by injecting and burning fuel in the hot cylinder volume. However, in order to replace the oxygen consumed by the combustion process, all of the prior art engines have required an auxilliary air compressor, supercharger or blower to introduce fresh air into the working fluid. The present invention shows that by changing from a two-stroke cycle to a four-stroke cycle the compressor or blower is not required; instead, every alternate revolution of the engine is used to intake fresh air and to exhaust spent combustion products.
In the prior art internal combustion Stirling engines different methods have been taught to dispose of waste heat. The second law of thermodynamics states that waste heat must be discharged from every engine which converts high temperature heat into work. This law of nature is sufficiently basic that machines which purport to do otherwise are sometimes called "perpetual motion machines of the second kind".
The Cowans engine and the experimental engine described by Hurley both include coolant passages or a conventional water-cooled heat exchanger for removing the waste heat. Thus these engines follow classic external combustion Stirling engine technology for waste heat removal, and suffer from the expense and complication of providing heat exchangers to transfer waste heat from the working fluid to the coolant, and radiators to reject the heat from the coolant.
The Hirsch engine discharges its waste heat without the use of either a heat exchanger or a radiator. It does this by injecting a spray of cooling water directly into the cold cylinder working volume. The water is vaporized by the heat of compression, and the water vapor, bearing the cycle waste heat is then exhausted with the spent combustion products.
While the Hirsch engine eliminates the requirement for cold side heat transfer equipment, it introduces the new requirement for large quantities of relatively pure cooling water, which would be an undesirable feature in an automotive power plant. The Pattas engine has no cold side heat exchanger, and does not inject cooling water, hence the waste heat rejection in the Pattas engine must be accomplished by heat transfer through the piston head and cylinder walls of the cold gas section, and/or by convecting heat out with the partial exchange of the working medium each revolution. Pattas teaches that only a partial exchange of the working medium is required each revolution, and the only reason stated for the exchange is the replenishment of oxygen required for combustion. When an engine is run according to this teaching, the heat rejection from the cold gas section is inadequate, causing the temperature to rise with a resulting reduction of power output and thermal efficiency. Thermodynamic calculations show that if an engine of this sort, with adiabatic walls, is run injecting only the minimum amount of air required for combustion, the cold space temperature will increase without bound, eventually reducing the power output and efficiency to zero. The present invention shows that in order to obtain high power output and efficiency without the requirement for cold space heat exchangers or water injection it is necessary to pass the maximum possible mass of air through the engine, so as to carry away the waste heat in the exhaust stream and thus maintain the cold cylinder working fluid at a low temperature level. This requirement is well met by the use of the four stroke cycle, in which each alternate revolution of the engine is devoted to exhausting one displacement volume of spent, warm working fluid, and intaking one displacement volume of cold fresh air.
A problem which must be addressed with any heat engine is control of the output torque or power. This is especially important in an automotive power plant, where the engine operates most of the time at a small fraction of full power, but must still maintain a high thermal efficiency. Rapidity of control is also very important. The Hirsch engine is equipped with a throttle valve in the regenerator duct, which would give a very rapid control, at the expense of efficiency, and also incorporates an air compressor and governor-regulated back-pressure valve, which control torque by varying the mean pressure level of the working fluid. The Cowans engine throttles the intake to the air compressor to produce the desired variation in the mean pressure of the working fluid. This decreases efficiency at part throttle because of the "pumping loss", which is the net compression work which must be done to raise the low pressure air behind the throttle valve to the atmospheric pressure at which it is eventually exhausted from the engines. A more loss-free method of controlling power in an open cycle Stirling engine is to vary the volume ratio (compression ratio) by varying the clearance volume. One way of doing this is by providing a series of appropriately sized volumes which can be connected to or isolated from the working volume by opening or closing valves. This method of power control has been used on closed cycle Stirling engines, and is particularly well suited to an open cycle Stirling engine. Another method of varying the compression ratio for power control is to locate a movable plunger or piston and cylinder so as to communicate with the working volume of the engine. Retracting the piston into its cylinder will increase the clearance volume of the engine, reduce the compression ratio, and reduce power. This method is especially appropriate to an engine which is at atmospheric pressure during a portion of each cycle, because relatively small forces are required to move such a control piston during this portion of the cycle. Since there are high pressure forces on the control piston during other portions of the cycle, it is desirable to position the control piston by machine elements which will not convey the periodic high forces back to the control mechanism. Such machine elements are well-known, and include screws or wormgears, wedges or cams with shallow angles and hydraulic cylinders with check valves. Control of power by variation of the compression ratio is limited because of the large volumes required for very large power variations; for this reason the technique is well suited for controlling cruising power, but it is not well suited for reducing power to idling levels. For this reason, an automotive powerplant should be equipped with the compression ratio control for efficient cruising power control, and with an intake or regenerator duct throttle valve to reduce power to idling values.
A regenerated internal combustion engine differs from a non-regenerated engine by its capability to extract unused high temperature heat from the spent working fluid and re-introduce it into the working fluid compressed for the succeeding cycle. Although this is a benefit in many ways, it also presents a problem of temperature control which is not found in non-regenerated engines. A conventional Otto engine starts anew each cycle by drawing in a charge of cold fresh air mixed with fuel vapor. This charge is compressed and burned, and attains a final burned temperature which depends upon the heating value of the fuel, the fuel/air ratio and the compression ratio. The final temperature attained during each particular cycle of operation depends only upon the values for these variables which were in effect during that particular cycle. Hence an Otto engine can never attain a working fluid temperature higher than the maximum which occurs at approximately the stoichiometric fuel/air ratio. If the engine hardware is built to withstand this gas temperature it should also be capable of withstanding the temperature at any other operating point. What is true for an Otto engine is also true for a Diesel engine, i.e. the peak gas temperature attained during each cycle depends upon the compression ratio, and fuel/air ratio, used for that particular cycle of operation, but does not depend upon what has taken place during previous cycles. The situation in a regenerated internal combustion engine is completely different. When the compressed air is passed through the regenerator, it is heated to the temperature which was reached by the previous charge of working fluid at the end of its expansion stroke. Fuel is then injected and burned to attain the peak cycle temperature, and the fluid is then expanded, which removes work and lowers its temperature. If the temperature increment due to the combustion of fuel is greater than the temperature decrement from the expansion, the final gas temperature which is stored in the regenerator will be higher than the temperature taken from the regenerator earlier in the cycle. If this imbalance of heat addition and work extraction persists for a considerable number of cycles the regenerator hot-end temperature can rise (or fall) without bound. Rising too far can cause the production of nitrogen oxides and can damage the regenerator. Falling too far can cause lowered efficiency, and impaired ignition of the fuel. Since the torque of the engine is controlled by compression ratio variation and by throttling, it is not obvious from the response of the engine when the temperature is rising or falling out of the desired range. In order to control the regenerator hot-end temperature a feedback controller must be employed, which senses the regenerator hot end temperature and increases or decreases the amount of injected fuel accordingly, so as to hold the temperature in the desired range. The temperature sensing method can be by high temperature thermocouple or by sensing the ratio of pressures before and after the regenerative heating event in the cycle. The sensing of the pressure ratio requires the use of complex electronics such as a microcomputer, but gives the fastest possible response to unwanted variations in temperature.